Sidebar 3: Measurements
I measured the Verity Audio Montsalvat DAC/PRE with my Audio Precision SYS2722 system (see the January 2008 "As We See It"), repeating some tests with the magazine's Audio Precision APx555 system. Apple's USB Prober utility identified the Verity as "Combo384 Amanero" from "Amanero Technologies" with the serial number string "413-001." The Verity's USB port operated in the optimal isochronous asynchronous mode, and Apple's AudioMIDI utility revealed that the DAC/PRE accepted 32-bit integer data sampled at all rates from 44.1kHz to 384kHz. The AES/EBU and coaxial S/PDIF inputs accepted 16- and 24-bit data sampled at rates up to 192kHz.
The DAC/PRE's output level at 1kHz in DAC mode was 3.43V from the balanced outputs, 2.1V from the single-ended outputs. In Preamp mode, the output with the volume control set to "0.0 dB" was the same as it was in DAC mode, but the maximum output level was now 13.28V, balanced, and 8.17V, unbalanced. With the Polarity button set to "+," both outputs preserved absolute polarity (ie, were noninverting) in both modes. The single-ended output impedance was less than 1 ohm at all audio frequencies. In contrast, the balanced output impedance was a very high 3.9k ohms at 20Hz and 1kHz, dropping slightly to 3.7k ohms at 20kHz. This would not have been a problem with the Parasound amplifiers I use for my auditioning, however, which have a very high input impedance.
The Verity offers a single reconstruction filter for 44.1kHz PCM data, which, as shown in fig.1, is a conventional linear-phase filter with symmetrical ringing before and after the single full-scale sample. This filter's ultrasonic rolloff with 44.1kHz data (fig.2, magenta and red traces) reaches full stop-band attenuation at 24kHz, a little above half the sample rate (vertical green line). The aliased image at 25kHz of a full-scale tone at 19.1kHz (cyan, blue) is suppressed by 120dB, and the harmonics associated with the 19.1kHz tone are all very low in level.
Fig.1 Verity DAC/PRE, impulse response (one sample at 0dBFS, 44.1kHz sampling, 4ms time window).
Fig.2 Verity DAC/PRE, wideband spectrum of white noise at –4dBFS (left channel red, right magenta) and 19.1kHz tone at 0dBFS (left blue, right cyan) into 100k ohms with data sampled at 44.1kHz (20dB/vertical div.).
Fig.3 shows the frequency response with data sampled at 44.1, 96, and 192kHz. The output is down by 0.5dB at the top of the audioband at all three sample rates, with the smooth ultrasonic rolloff interrupted by a steep rolloff just below half the sample rate with 96kHz and 192kHz data. Channel separation was superb, at >125dB in both directions below 1kHz, decreasing to a still superb 113dB at 20kHz. The DAC/PRE's noise floor was free from any power supply-related spuriae (fig.4).
Fig.3 Verity DAC/PRE, frequency response at –12dBFS into 100k ohms with data sampled at: 44.1kHz (left channel green, right gray), 96kHz (left cyan, right magenta), and 192kHz (left blue, right red) (1dB/vertical div.).
Fig.4 Fig.4 Verity DAC/PRE, spectrum with noise and spuriae of dithered 1kHz tone at 0dBFS with 24-bit data (left channel blue, right red) (20dB/ vertical div.).
An increase in bit depth from 16 to 24, with dithered data representing a 1kHz tone at –90dBFS, dropped the Verity's noise floor by around 27dB (fig.5). This implies a resolution of >20 bits, which is among the highest I have encountered. When I played undithered data representing a tone at exactly –90.31dBFS, the waveform was symmetrical, with negligible DC offset and the three DC voltage levels described by the data free from noise (fig.6). With undithered 24-bit data (fig.7), the DAC/PRE's very low analog noise floor means it can output a clean sinewave, even at this very low signal level. Linearity error with 24-bit data (not shown) was very low down to below –120dBFS.
Fig.5 Verity DAC/PRE, DAC mode pre-repair, spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS with: 16-bit data (left channel cyan, right magenta), 24-bit data (left blue, right red) (20dB/vertical div.).
Fig.6 Verity DAC/PRE, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit data (left channel blue, right red).
Fig.7 Verity DAC/PRE, waveform of undithered 1kHz sinewave at –90.31dBFS, 24-bit data (left channel blue, right red).
All the previous measurements were taken with the Verity in DAC mode before the intermittent fault that I mention in the review text raised its head. When I received the repaired DAC/PRE back from Verity, I repeated some of the testing. However, the results in DAC mode were not as good as they had been before the repair. Fig.8, for example, repeats the spectral analysis taken with a dithered 16- and 24-bit 1kHz tone at –90dBFS. Compared with the spectra in fig.5, the 24-bit noise floor (blue and red traces) has risen by around 6dB and the spectrum of the tone now has a pair of low-frequency sidebands visible around each of the harmonic frequencies. Switching the Verity to Preamp mode with the volume control set to "0.0dB" and repeating this test gave the same excellent result shown in fig.5. I therefore continued the testing of the repaired DAC/PRE in Preamp mode with the volume control set to "0.0dB."
Fig.8 Verity DAC/PRE, DAC mode post-repair, spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS with: 16-bit data (left channel cyan, right magenta), 24-bit data (left blue, right red) (20dB/vertical div.).
In this mode, the DAC/PRE produced very low levels of harmonic distortion with full-scale data (fig.9). The third harmonic is the highest in level, at just –120dB (0.0001%), and all other harmonics are even lower in level. This spectrum was taken into the high 100k ohms load. When I reduced the load impedance to the punishing 600 ohms, the levels of the harmonics didn't rise. Intermodulation distortion in Preamp mode with an equal mix of 19 and 20kHz tones, each lying at –6dBFS, was similarly very low (fig.10), with the difference tone at 1kHz lying at –124dB (0.00006%).
Fig.9 Verity DAC/PRE, 24-bit data, spectrum of 50Hz sinewave, DC–1kHz, at 0dBFS into 100k ohms (left channel blue, right red; linear frequency scale).
Fig.10 Verity DAC/PRE, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 0dBFS into 100k ohms, 24-bit, 44.1kHz data (left channel blue, right red; linear frequency scale).
I examined the DAC/PRE's rejection of word-clock jitter via its USB, AES/EBU, and S/PDIF inputs. Fig.11 shows the spectrum of the DAC/PRE's output when it was fed high-level, AES/EBU 16-bit J-Test data. All the odd-order harmonics of the undithered low-frequency, LSB-level squarewave lie at the correct levels, indicated by the sloping green line, but some closely spaced sidebands can be seen surrounding these harmonics. There is also a trio of spurious tones visible between 9kHz and 10kHz, though these are low in level. These tones can be seen more clearly with 24-bit USB data (fig.12).
Fig.11 Verity DAC/PRE, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit AES/EBU data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
Fig.12 Verity DAC/PRE, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 24-bit USB data sourced from MacBook Pro (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
I shall assume that the less-than-superb performance I experienced in DAC mode was specific to this much-traveled sample. But in Preamp mode, Verity's DAC/PRE joins that small group of D/A processors that offers close-to–state-of-the-art resolution, joining Benchmark's DAC3, Chord's DAVE, exaSound's s88, HoloAudio's May, MBL's N31, Okto Research's dac8 Stereo, and the Weiss DAC502.—John Atkinson
